1. Field of the Invention
The present invention relates to a method of removing oxide scale generated on the surface of titanium material and descaled titanium material.
2. Description of the Related Art
In manufacture of plates, tubes, bars, and wires from titanium or titanium alloy (hereinafter collectively called "titanium material"), oxide scale generated on the surface of the material in the process of annealing or a like step is required to be removed. The most commonly used method of removing scale is a method in which titanium material is subjected to acid pickling by use of a mixed acid solution composed of nitric acid and hydrofluoric acid (hereinafter referred to as "nitric-hydrofluoric acid pickling solution").
However, if scale is thick, or in a case where thick scale is generated on a titanium plate in the process of annealing performed after hot rolling, the scale cannot be completely removed merely through acid pickling by use of a nitric-hydrofluoric acid pickling solution. Therefore, for example, descaling of a titanium plate having a thickness of 3 mm or more is often performed according to a method in which the plate is subjected to acid pickling after a mechanical descaling treatment such as shot blast treatment.
As in the case of ordinary stainless steel strips, cold-rolled titanium strips are successively subjected to annealing and descaling in a successive annealing-pickling line in order to improve production efficiency.
Generally, an annealing furnace is a combustion furnace of a tunnel type which employs hydrocarbon gas as fuel. Since a strip is heated to about 700-800.degree. C. while being passed through the interior of the furnace, oxide scale is generated on the surface of the strip. For removal of the oxide scale from the surface of the strip after annealing, the strip is first immersed in a fused alkaline salt bath, and then subjected to acid pickling by use of a nitric-hydrofluoric acid pickling solution.
Since a cold-rolled titanium strip generally has a relatively small thickness of 2 mm or less, if the strip is subjected to a mechanical descaling treatment such as shot blast treatment, large residual deformation or warp is imparted to the strip. Therefore, the strip is subjected to a fused alkaline salt bath treatment instead of a mechanical treatment.
Fused alkaline salts generally used in such a salt bath comprise a mixture of alkali mainly composed of sodium hydroxide and sodium nitrate, and salts, which mixture is heated and fused at 430-550.degree. C. when used.
For example, Japanese Patent Publication (kokoku) No. 4-72914 discloses, as a method of removing oxide scale from the surface of a cold-rolled titanium plate, a method in which a plate is immersed in a fused alkaline salt bath mainly containing sodium hydroxide and an oxidizing agent; and the plate is subjected to acid pickling by use of a nitric-hydrofluoric acid pickling solution. However, in this method involving immersion of a titanium plate in a fused alkaline salt bath, sparks are likely to be generated and create flaws on the surface of the titanium plate in the bath (such flaws are hereinafter called "spark flaws").
In a fused alkaline salt bath, spark flaws are generated due to a potential difference between a titanium strip and an iron-made immersion roll for immersing the strip in the fused alkaline salt bath. In other words, a spark is discharged at the moment the titanium strip having dissolved oxide scale on its surface contacts the iron-made immersion roll so that the surface of the titanium strip is fused locally, resulting in spark flaws.
Japanese Patent Application Laid-Open (kokai) No. 3-247785 discloses a method of descaling a titanium strip in which generation of the above-described spark flaws can be prevented. In this method, a potential difference between a titanium strip and an iron-made immersion roll is reduced by means of a titanium-made sacrificial anode which forms a short circuit between the anode and the iron-made immersion roll in a fused alkaline salt bath.
In this method, however, since titanium of the sacrificial anode dissolves into the fused alkaline salt bath, deterioration of the bath is disadvantageously accelerated, and expensive titanium is wastefully consumed.
Japanese Patent Application Laid-Open (kokai) No. 4-45293 discloses a method in which generation of spark flaws is prevented by arranging above the surface of the bath an entry-side immersion roll immersed in a fused alkaline salt bath and an exit-side immersion roll.
In this method, oxide scale on the surface of the titanium plate prevents contact between the titanium plate and the entry-side immersion roll, resulting in no spark discharge, and even if oxide scale is dissolved in the bath and no oxide scale exists at the exit side of the bath, no electric cell is formed at the exit-side immersion roll since the roll is above the surface of the bath, resulting in no spark discharge.
However, this method is difficult to apply to actual operations, since the heights of immersion rolls must be regulated depending on the thickness and dissolution rate of scale. The above-described problems of generation of spark flaws hinder sufficient treatment by use of fused alkaline salts, resulting in scale remaining after descaling treatment.
Japanese Patent Application Laid-Open (kokai) No. 56-15679 discloses a method of electrolytic descaling of a titanium sheet in which a polishing tool is slid on the surface of a titanium sheet serving as an cathode, to thereby perform anodic electrolysis.
However, since this method is a method of descaling through dissolving of the titanium base metal by use of a hydrochloric acid solution serving as an electrolyte solution, the surface of a titanium sheet becomes rough after descaling, failing to produce a titanium sheet having excellent surface finish. Also, this method requires installation of a polishing tool, such as a polishing belt, in an electrolysis vessel, leading to high equipment cost.
Japanese Patent Application Laid-Open (kokai) No. 60-194099 discloses a method of removing oxide scale from titanium material through electrolytic pickling by use of an aqueous solution of nitric-hydrofluoric acid. This method aims to shorten the time required for acid pickling.
Although the time for acid pickling can be shortened by this method, expensive material such as platinum must be used for electrodes, since a nitric-hydrofluoric acid solution is highly corrosive.
Japanese Patent Application Laid-Open (kokai) No. 2-310399 discloses a method of removing oxide scale from titanium material in which electrolysis is performed while a cloth soaked with a sulfuric acid electrolyte solution is caused to contact titanium material to be treated. However, this method is established in order to immediately and safely remove oxide scale locally generated in the course of welding or a like process. If this method is applied to a titanium strip having a large area, gas generated during electrolysis assumes the form of bubbles, and the bubbles accumulate in spaces between the cloth soaked with the electrolyte solution and the surface of the titanium strip. As a result, the area of the strip in contact with the electrolyte solution is decreased so that efficiency of descaling is lowered and uniformity of descaling is lost. Therefore, surface roughness and surface gloss become disadvantageously nonuniform.
Conventionally, polishing techniques such as mechanical, chemical, and electrolytic polishings have been known for producing titanium material of a low surface roughness and excellent surface gloss. If these techniques are applied to the production of titanium material, the surface roughness of the resultant material may be as low as or lower than that of titanium material descaled by the method of the present invention. However, in order to apply these techniques, expensive equipment and chemicals are required, and high labor cost and manufacture know-how are also required, resulting in greatly increased costs in manufacture. Although rolls having a low surface roughness may be used for finish-rolling to provide titanium material with a low surface roughness, crystal grains in the material will be deformed as the reduction ratio increases, which results in hardening and poor formability of the material.